(1) Field of the Invention
The present invention relates to apparatus for determining the relative displacement between two relatively movable members.
In particular, the invention relates to an angular displacement transducer, having an output signal dependent on the angular position of a rotary member relative to a fixed member, typically the transducer housing. The output signal is generated even when there is no relative movement between the members.
(2) Description of the Prior Art
In a known transducer, the rotary member is a rotatable disc carrying an angularly coded track. The track is made up of a sequence of light and dark features, forming a binary code. The code on the disc has the property that any group of N adjacent bits is unique, so that by reading N bits of code, the angular position of the disc can be determined. Typically N is of the order of 15 for adequate resolution of the angular position.
An angularly fixed read head is used to read the code on a small area of the disc. The read head comprises N adjacent photosensitive devices, since the position to be determined requires that N bits of code must be read from the disc.
A disadvantage of this type of transducer is that the arrangement, particularly the optical arrangement for reading the N bits of code simultaneously, is complicated and therefore expensive. If more resolution of the angular position is required, more bits of code need to be read, requiring a more complicated and expensive optical read head. Furthermore, if it is desired to bury the angular code in a subcode as is often desirable, even more bits of the coded track will need to be read simultaneously to obtain an angular position determination. This requires an even more complicated, and therefore more expensive, read head.